1. Contribution from the Department of Chemistry, Institute of Chemical Technology II, Darmstadt University of Technology, Petersenstrasse 20, D-64287 Darmstadt, Germany, Department of Catalysis, Institute for Applied Chemistry, Richard-Willstätter-Strasse 12, D-12489 Berlin, Germany, and Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany